1. Field of the Invention
The invention relates to an improvement in the design of axial gap slotless electric motors and generators by increasing the torque that can be obtained for a given radial size rotor, and to reducing the size and weight of such a machine by using tapered coil conductors; and to improving the cooling capacity of the machine.
2. Description of Related Art
Electric motors and generators of the axial gap type are well known. They employ motors that deliver axially directed magnetic fields to stationary radially directed electrically conductive coils of wire. It is common to have rotors with permanent magnets of alternate polarity with the magnets of opposite rotors attracting each other, concentrating the magnetic field across a stator sandwiched in between the permanent magnet rotors. In the motor mode, electric current is passed through the stator windings. Currents flowing through the radially positioned conductors intersect the axial field created by the magnets to produce a torque that rotates the motor rotor. In the generator mode the magnets are rotated by an external prime mover. When this happens, the rotating magnet's axial field interacts with the radial conductor coils to induce a current flow in and voltage across the conductor coils. FIG. 1 illustrates a prior art configuration showing the direction of current, magnetic field and resulting forces. FIG. 2 illustrates the prior art phase coil arrangement. The phase coil is a continuous conductor wound flat in a closed loop form. It generally consists of "active sectors" or areas (i.e., those portions of the coil that extend radially out from and perpendicular to the axis of rotation of the rotor and that cross the magnetic field created by the magnets and "inactive sectors" or areas (i.e. those portions of the coil that are outside the magnetic field which connect the active sectors). The current in the active portion of the coils flows radially outwardly on one side and radially inwardly on the other or opposite side of the coil.
FIG. 1 illustrates the interaction of the magnetic fields and currents under consideration and in the prior art axial gap motor 10. Two rotating permanent magnet disks 1, 2 sandwich a stator ring 3 separated by two air gaps 4, 5 as they rotate about an axis 7. The rotating magnet rings 1, 2 have alternately poled flat permanent magnets, typically of sector shape, which produce an axially directed magnetic field 6 across the air gaps. The stator ring 3 contains sets of phase coils. Radially directed current 8 in the active part of the coil conductors interacts with the axially directed magnetic field 6 to produce a tangentially directed force 9 which creates the torque that drives the motor.
FIG. 2 illustrates a typical arrangement of phase coils 23 on a stator ring 21 of a stator 20 having an opening 25 that accommodates a rotor that rotates about an axis of rotation 7. In the coils the conductor wires form loops with the wires formed into an essentially inner radius or sector 27 and outer radius or sector 26 with radial conductor wires 22 extending between the inner and outer radii of the coil. The standard uniform conductor wires used leave vacant areas 24 within and between the coils that are often overlapped. These empty areas are not utilized for generating torque. When using uniform cross-section conductors these "empty areas" equal 1.sub.r /(2r.sub.i) fraction of the conductor area where 1.sub.r =active length of conductor 22 under the rotating magnets and r.sub.i =inner radius 27 of the coil. For a typical design with 1.sub.r =r.sub.i', this implies that 50% of the space available for conductors in the stator is not used to produce torque. This ratio of "empty area" to "conductor area" is called the "spoke fill factor." This loss of useful area is unique to axial motors. It results in a loss in torque that is in addition to the conventional "shape fill factor", that defines the loss of area due to the shape of the wire, and the "insulation fill factor", that defines the loss of area due to the insulation coating on the bare copper wire. The combination of "shape fill factor" and "insulation fill factor" is called the "copper fill factor." As an example, round wires have a "copper fill factor" of 0.7. This loss coupled with a "spoke fill factor" of 50% yields a total fill factor of (0.7)(0.5)=0.35 or 35% for round wires. The situation is worse if Litz wires are used. Litz wires have typical "copper fill factors" of 0.4 to 0.55. This coupled with the "spoke fill factor" of 50% can yield a still lower total fill factor of (0.4)(0.5)=0.20 or 20%. This implies that only 20% of the available area is used to produce torque. As a result, the torque produced is significantly lower than what is possible when the entire available geometric area is used to produce torque.
As to specific details, the use of tapered conductors for armatures is old with Edison, U.S. Pat. No. 242,898, issued Jun. 14, 1881, and Apple, U.S. Pat. No. 1,631,186, issued Jun. 7, 1927, examples, as is tapered conductors for general electrical conduction in rotary electric machines with Henry-Baudot, U.S. Pat. No. 3,223,870, issued Dec. 14, 1965, and Kanayama et al, U.S. Pat. No. 4,484,097, issued Nov. 20, 1984, examples. Shildneck, U.S. Pat. No. 3,014,139, issued Dec. 19, 1961 is an example of tapered conductors combined and used to conduct coolant through electrical machinery. The patents by Edison. Henry-Baudet, and Kanayama, use individual radiating plates; these individual plates are joined electrically at their ends for electrical transfer only.
The basic theories pertaining to, and several illustrations and versions of, axial gap machines are set forth in an article by P. Campbell in Proceedings of the Institute of Electrical Engineers, Vol. 121, No. 12. December 1974, pages 1489-1494, and an article titled "Principles of a Permanent-Magnet Axial-field d.c. Machine," by P. Profumo, Z. Zhang and A. Tencolni in IEEE Transactions on Industrial Electronics, Vol. 44, No. 1, February 1997, pages 29-45, which are incorporated herewith by reference.